1. Field of the Invention
The present invention relates to a fundus camera that includes a light-emitting diode for emitting white light as a light source for illuminating a fundus of an eye to be examined.
2. Description of the Related Art
Conventionally, a halogen lamp that emits steady light as an observation light source and a xenon tube that emits pulse light as a photographing light source have been used as a light source of a fundus camera.
Further, an illumination device which uses a white light-emitting diode (LED) as alight source has come to be used instead of a fluorescent lamp or an incandescent lamp. The white LED emits white light and consumes only a small amount of power and generates a small amount of heat. Also in an ophthalmologic apparatus such as a fundus camera, an infrared LED has been used as an auxiliary light source or an index projecting light source for alignment.
Further, a white LED has been employed as a light source of a medical device such as an endoscope. The following patent documents discuss an apparatus which uses white light emitted from a white LED.
In Japanese Patent Application Laid-Open No. 2006-174984, light flux which has a plurality of emission colors is emitted from a plurality of LEDs. The LEDs are disposed in a ring shape toward a concave diffuse-reflective member, and diffused and reflected light flux is brought out from a ring-shaped opening, so that efficiency is improved. Further, the LED has white light and infrared light, and an LED which emits light according to a photographing mode is configured to be switched.
In Japanese Patent Application Laid-Open No. 2007-29726, a plurality of LEDs which are disposed in a ring shape and have different wavelengths is used as a light source of a fundus camera, and has at least one of white light, green light, blue light, and infrared light. Further, at least one of the LEDs is used as a flash light source.
Japanese Patent No. 3409666 discusses a planar light emitting apparatus that irradiates a fluorescent member with blue light emitted from a blue LED. The light emitting apparatus mixes the blue light emitted from the blue LED with fluorescence emitted from the fluorescent member to emit white light.
As described above, a white LED which emits white light has come to be used as an observation or a photographing light source in an ophthalmic photographing apparatus such as a fundus camera.
In general, a white LED emits white light by irradiating a yellow fluorescent member with light emitted from a blue LED and mixing the light emitted from the blue LED with the fluorescence emitted from the fluorescent member. Since a part of blue light emitted from the blue LED may be directly emitted without being absorbed by the fluorescent member, a spectral intensity distribution of the white LED is formed as illustrated in FIG. 10. The intensity of green light and red light is lower than that of blue light, so that the color of light becomes pale white. Accordingly, there is a problem in that a color rendering property is lowered.
If photographing is performed using the above-described white LED as a photographing light source, color development is bad in comparison with a conventional halogen lamp or a xenon tube. The color of an eye fundus as an object includes many red components and few blue components. Therefore, an image of the eye fundus shows an unnatural color and use efficiency of the light also deteriorates.
This is because an amount of light which passes through the fluorescent member without exciting the fluorescent member is larger than an amount of light which is emitted from the excited fluorescent member as for the blue light emitted from the blue LED. If the above-described light source is used, generally a known method for maintaining a balance in a color by a balancing filter has been employed. However, in this case, the use efficiency of the light deteriorates.
In recent years, a white LED which has a high color rendering property has been in practical use, in which several kinds of fluorescent members are mixed or an LED emitting a near-ultraviolet ray and a fluorescent member excited by the near-ultraviolet ray are used. Even in the case of the white LED which has a high color rendering property, some light passes through the fluorescent member without exciting the fluorescent member as for light emitted from the LED and energy use efficiency is low.